Polypropylene polymeric compositions have gained wide commercial acceptance and usage in numerous commercial applications because of the relatively low cost of the polymers and the desirable properties they exhibit. In general, polypropylene polymers, particularly propylene homopolymers, have a disadvantage of being brittle with low impact resistance, especially at low temperatures. Numerous procedures have been proposed for modifying the properties of the propylene homopolymers to improve the impact strength and other low temperature properties. Many if not most of such proposals have included the provision of a propylene/.alpha.-olefin copolymer portion in otherwise homopolymeric polypropylene. The structure of such modified polypropylene polymers is variable, but somewhat uncertain. In Leibson et al, U.S. Pat. No. 3,514,501, there is described a process for the production of block copolymers wherein, for example, a prepolymer which is homopolymeric polypropylene is produced and a block of at least one .alpha.-olefin is grown from the prepolymer in a second polymerization step. A second approach to providing improved impact strength comprises the mixing of a polypropylene homopolymer with a propylene/ethylene copolymer. Most proposals, and most commercial products of high impact strength, result from the production of a first polymer, usually a polypropylene homopolymer, and the production of a copolymeric portion in the presence of the initial polymer product mixture which still contains active polymerization sites. Whether the resulting product is a block copolymer, a mixture of homopolymer and copolymer or is of other structure is not entirely clear. However, such products are well known and conventional in the art and have achieved considerable commercial importance. They are often referred to as polypropylene impact copolymers, regardless of the precise nature of their structure, and are said to contain a homopolymer phase (often homopolymeric polypropylene) and a rubber phase (the copolymer portion).
While such a polypropylene impact copolymer does exhibit improved low temperature impact strength and other improved low temperature properties as well as many of the desirable properties of the homopolymer such as stiffness, the impact copolymer compositions may exhibit considerable stress whitening under circumstances such as a) ejecting parts from molds (knock-out pin marks); b) the forming of shaped articles from sheet stock, etc., at temperatures in the vicinity of melting and below; and c) the general situation of impacting or bending of fabricated parts either during their production or assembly, or in their intended application.
A number of procedures have been described which are designed to produce compositions having good low temperature properties and better resistance to stress whitening. Many of these procedures include an extra processing step or the inclusion within the composition of an extra component. In European Patent Application 208,330 there are disclosed polymeric polypropylene compositions said to have improved resistance to stress whitening which comprise homopolymeric polypropylene or peroxide-degraded polypropylene which have a grafted ethylene/propylene copolymer portion, and as an additional component, an ester derived from a monocarboxylic acid of 12-20 carbon atoms and a polyhydric alcohol. In Japanese Patent J58210949A there is described a mixture of polypropylene of specified melt flow and an ethylene/.alpha.-olefin copolymer wherein the molecular weight of the polypropylene has been reduced substantially by treatment with peroxide. The compositions are said to have resistance to stress whitening as well as improved gloss and transparency, although in many other instances the reduction of molecular weight by peroxide treatment, a process known as visbreaking, served to increase rather than decrease stress whitening. It would be of advantage to provide improved polypropylene compositions characterized by good low temperature properties such as impact strength and also good resistance to stress whitening.